Hot-rolled galvanized steel sheets (such as hot-rolled hot-dip galvanized steel sheets and hot-rolled hot-dip galvanized steel sheets) are manufactured by coating hot-rolled steel sheets, and thus are economical as compared with the case of coating cold-rolled steel sheets because a cold rolling process and an annealing process are not required for producing the hot-rolled galvanized steel sheets.
In general, hot-rolled hot-dip galvanized steel sheets and hot-rolled hot-dip galvannealed steel sheets are manufacturing from hot-rolled steel sheets by pickling the hot-rolled steel sheets to remove scale formed during a hot rolling process, heating the hot-rolled steel sheets to a temperature of 480° C. to 550° C., which is slightly higher than the temperature of a galvanizing bath, and coating the hot-rolled steel sheets. Consequently, coating failure caused by surface concentration of alloying elements, or coating defects such as peeling of a coating layer are not present on the hot-rolled hot-dip galvanized steel sheets and hot-rolled hot-dip galvannealed steel sheets.
However, if austenitic high-manganese hot-rolled steel sheets (refer to Patent Documents 1 to 4) having strength and ductility markedly improved by adding manganese in an amount of 5% to 35% to form twin crystals during plastic deformation are coated, easily oxidizable alloying elements such as aluminum (Al) and silicon (Si) as well as manganese (Mn) are selectively oxidized when the austenitic high-manganese hot-rolled steel sheets are heated to a temperature of 480° C. to 550° C. for hot-dip galvanizing, and thus, thick oxide films are formed thereon to result in coating failure and peeling of a coating layer.
To remove such an oxide film from a high-manganese hot-rolled steel sheet, the high-manganese hot-rolled steel sheet may be pickled and heated under a nitrogen atmosphere containing hydrogen so as to activate the surface of the high-manganese hot-rolled steel and increase the temperature of the high-manganese hot-rolled steel to a value suitable for a coating process. The nitrogen atmosphere containing hydrogen reduces oxidation of iron (Fe, the steel sheet) and easily oxidizable elements such as manganese (Mn), silicon (Si), and aluminum (Al).
Therefore, if high-manganese steel including large amounts of aluminum and silicon in addition to a large amount of manganese is heated under such an atmosphere, such alloying elements may be selectively oxidized by a small amount of moisture or oxygen included in the atmosphere, and thus, manganese, aluminum, and silicon oxides may be formed on the high-manganese steel (a material to be coated).
That is, if a high-manganese steel sheet including large amounts of aluminum and silicon as well as a large amount of manganese is coated, since oxides are formed on the surface of the high-manganese steel sheet during a heat treatment process before a coating process, coating failure may be caused, or a coating layer may be peeled from the high-manganese steel sheet in a later processing process.
Patent Document 5 discloses a technique for preventing coating failure when a high-manganese hot-rolled steel sheet is coated by a hot-dip galvanizing method. According to Patent Document 5, surface scale is removed from a high-manganese hot-rolled steel sheet with an acid solution, and a nickel (Ni) plating layer is continuously formed on the high-manganese hot-rolled steel sheet to a density of 50 mg/m2 to 100 mg/m2, so as to prevent alloying elements such as manganese, aluminum, and silicon from concentrating on the surface of the high-manganese hot-rolled steel sheet in a later heat treatment process.
However, although being effective in preventing surface concentration of silicon and manganese, the nickel plating layer, formed before the heat treatment process, is not effective in preventing surface concentration of aluminum. Moreover, the nickel plating layer may facilitate diffusion of aluminum to result in an aluminum oxide (Al—O) on the nickel plating layer, and thus, coating failure or peeling of a coating layer may be caused.
That is, when a high-manganese hot-rolled steel sheet including a large amount of manganese is coated, since oxides of manganese, aluminum, silicon, or a combination thereof are formed on the steel sheet to a large thickness during a heat treatment process, coating failure may occur in a later hot-dip galvanizing process, or a coating layer may be peeled from the steel sheet in a later processing process. Therefore, technology for solving this limitation is required.
(Patent Document 1) Japanese Patent Application Laid-open Publication No. H4-259325
(Patent Document 2) International Patent Publication No. WO93/013233
(Patent Document 3) International Patent Publication No. WO99/001585
(Patent Document 4) International Patent Publication No. WO02/101109
(Patent Document 5) Korean Patent Application Laid-open Publication No. 2010-0007400